Beneficial Effect of Spironolactone Administration on Ethynylestradiol-Induced Cholestasis in the Rat: Involvement of Up-Regulation of Multidrug Resistance-Associated Protein 2

Multidrug Resistance-Associated Protein 2 Deficiency Aggravates Estrogen-Induced Impairment of Bile Acid Metabolomics in Rats

Multidrug resistance-associated protein 2 (Mrp2) mediates biliary secretion of anionic endobiotics and xenobiotics. Genetic alteration of Mrp2 leads to conjugated hyperbilirubinemia and predisposes to the development of intrahepatic cholestasis of pregnancy (ICP), characterized by increased plasma bile acids (BAs) due to mechanisms that are incompletely understood. Therefore, this study aimed to characterize BA metabolomics during experimental Mrp2 deficiency and ICP. ICP was modeled by ethinylestradiol (EE) administration to Mrp2-deficient (TR) rats and their wild-type (WT) controls. Spectra of BAs were analyzed in plasma, bile, and stool using an advanced liquid chromatography–mass spectrometry (LC–MS) method. Changes in BA-related genes and proteins were analyzed in the liver and intestine. Vehicle-administered TR rats demonstrated higher plasma BA concentrations consistent with reduced BA biliary secretion and increased BA efflux from hepatocytes to blood via upregulated multidrug resistance-associated protein 3 (Mrp3) and multidrug resistance-associated protein 4 (Mrp4) transporters. TR rats also showed a decrease in intestinal BA reabsorption due to reduced ileal sodium/bile acid cotransporter (Asbt) expression. Analysis of regulatory mechanisms indicated that activation of the hepatic constitutive androstane receptor (CAR)-Nuclear factor erythroid 2-related factor 2 (Nrf2) pathway by accumulating bilirubin may be responsible for changes in BA metabolomics in TR rats. Ethinylestradiol administration to TR rats further increased plasma BA concentrations as a result of reduced BA uptake and increased efflux via reduced Slco1a1 and upregulated Mrp4 transporters. These results demonstrate that Mrp2-deficient organism is more sensitive to estrogen-induced cholestasis. Inherited deficiency in Mrp2 is associated with activation of Mrp3 and Mrp4 proteins, which is further accentuated by increased estrogen. Bile acid monitoring is therefore highly desirable in pregnant women with conjugated hyperbilirubinemia for early detection of intrahepatic cholestasis.

The Effects of Spironolactone in Preventing Bile Duct Ligation-induced Hepatitis in A Rat Model

Cholestasis is associated with the accumulation of bile acids and bilirubin in the hepatocytes and leads to liver injury. Pregnane X Receptor (PXR) coordinates protective hepatic responses to toxic stimuli, and this receptor was reported to stimulate bile secretion by increasing MRP2 expression. Since PXR activators were reported to be anti-inflammatory in the liver, PXR was proposed as a drug target for the treatment of chronic inflammatory liver diseases. We investigated the potential protective effect of spironolactone (SPL), an enzyme inducer, in hepatotoxicity induced by bile duct ligation in rats. Wistar Albino (250-300 g) rats were divided into the control group and the bile duct ligated (BDL) group. BDL group was divided into three subgroups; following BDL, for 3 days, the first group received propylene glycol (vehicle of SPL) (blinded), the second subgroup received spironolactone (SPL) (200 mg/kg oral), and the third subgroup received SPL for 3 days, starting 3 days after the bile duct ligation, in order to investigate if it has a healing effect after hepatitis had developed. The control group was sham-operated and received saline. At the end of the experiment, blood and tissue samples were collected. Serum TNF-α, NF-ĸB, bilirubin, IL-6 levels, ALT, AST, ALP activities and tissue MPO activity and oxidant damage increased after the bile duct ligation was significantly decreased following SPL administration. PXR and MRP2 activity showed an increase in the hepatocytes as a result of the treatment. In conclusion, it was observed that SPL administration significantly decreases liver inflammation and damage related to BDL.

Rifampicin induces clathrin-dependent endocytosis and ubiquitin-proteasome degradation of MRP2 via oxidative stress-activated PKC-ERK/JNK/p38 and PI3K signaling pathways in HepG2 cells

It was reported that antituberculosis medicines could induce liver damage via oxidative stress. In this study, we investigated the effects of rifampicin (RFP) on the membrane expression of multidrug resistance-associated protein 2 (MRP2) and the relationship between oxidative stress and RFP-induced endocytosis of MRP2 in HepG2 cells. We found that RFP (12.5–50 μM) dose-dependently decreased the expression and membrane localization of MRP2 in HepG2 cells without changing the messenger RNA level. RFP (50 μM) induced oxidative stress responses that further activated the PKC-ERK/JNK/p38 (protein kinase C-extracellular signal-regulated kinase/c-JUN N-terminal kinase/p38) and PI3K (phosphoinositide 3-kinase) signaling pathways in HepG2 cells. Pretreatment with glutathione reduced ethyl ester (2 mM) not only reversed the changes in oxidative stress indicators and signaling molecules but also diminished RFP-induced reduction in green fluorescence intensity of MRP2. We conducted co-immunoprecipitation assays and revealed that a direct interaction existed among MRP2, clathrin, and adaptor protein 2 (AP2) in HepG2 cells, and their expression was clearly affected by the changes in intracellular redox levels. Knockdown of clathrin or AP2 with small interfering RNA attenuated RFP-induced decreases of membrane and total MRP2. We further demonstrated that RFP markedly increased the ubiquitin–proteasome degradation of MRP2 in HepG2 cells, which was mediated by the E3 ubiquitin ligase GP78, but not HRD1 or TEB4. In conclusion, this study demonstrates that RFP-induced oxidative stress activates the PKC-ERK/JNK/p38 and PI3K signaling pathways that leads to clathrin-dependent endocytosis and ubiquitination of MRP2 in HepG2 cells, which provides new insight into the mechanism of RFP-induced cholestasis.

Spironolactone in Combination with α-glycosyl Isoquercitrin Prevents Steatosis-related Early Hepatocarcinogenesis in Rats through the Observed NADPH Oxidase Modulation

Administration of the diuretic, spironolactone (SR), can inhibit chronic liver diseases. We determined the effects of SR alone or in combination with the antioxidant α-glycosyl isoquercitrin (AGIQ) on hyperlipidemia- and steatosis-related precancerous lesions in high-fat diet (HFD)-fed rats subjected to a two-stage hepatocarcinogenesis model. Rats were fed with control basal diet or HFD, which was administered with SR alone or in combination with an antioxidant AGIQ in drinking water. An HFD increased body weight, intra-abdominal fat (adipose) tissue weight, and plasma lipids, which were reduced by coadministration of SR and AGIQ. SR and AGIQ coadministration also reduced hepatic steatosis and preneoplastic glutathione S-transferase placental form-positive foci, in association with decrease in NADPH oxidase (NOX) subunit p22phox-positive cells and an increase in active-caspase-3-positive cells in the foci. Hepatic gene expression analysis revealed that the coadministration of SR and AGIQ altered mRNA levels of lipogenic enzymes ( Scd1 and Fasn), antioxidant-related enzymes ( Catalase), NOX component ( P67phox), and anti-inflammatory transcriptional factor ( Pparg). Our results indicated that SR in combination with AGIQ had the potential of suppressing hyperlipidemia- and steatosis-related early hepatocarcinogenesis through the reduced expression of NOX subunits.

Pharmacokinetic Characteristics of Baicalin in Rats with 17α-ethynyl-estradiol-induced Intrahepatic Cholestasis

Baicalin is one of the main active ingredients of choleretic traditional Chinese medicine drug Radix Scutellariae. The aim of this study was to explore the pharmacokinetic characteristics of baicalin in rats with 17α-ethynylestradiol (EE)-induced intrahepatic cholestasis (IC) based on its choleretic effects. Firstly, rats were subcutaneously injected with EE solution (5 mg/kg, 0.25 mL/100 g) for 5 consecutive days to construct an IC model. Then the bile excretion rate, serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP) and total bile acid (TBA) and pathological changes of the liver were detected. Secondly, after successfully modeling, the rats were intragastrically given baicalin solution (200 mg/kg) (n=6). Blood samples were collected from the tail vein at different time points after intragastric administration. The protective effects of low- (50 mg/kg), medium- (100 mg/kg) and high-dose (200 mg/kg) baicalin on the liver in IC rats were evaluated. The content of baicalin in plasma was detected by liquid chromatography-mass spectrometry/mass spectrometry and pharmacokinetics parameters were calculated. Pharmacodynamic results showed that low-, medium- and high-dose baicalin all significantly increased the average excretion rate of bile (P<0.05), and significantly decreased serum levels of ALT, AST and ALP and TBA (P<0.05). Meanwhile, HE staining showed that baicalin significantly relieved EE-induced hepatocyte edema and necrosis. Pharmacokinetic results exhibited that the absorption of baicalin in both IC and normal control rats showed bimodal phenomenon. C_max, AU_(0-t) and AUC_(0-∞) of baicalin in IC rats were significantly higher than those of the normal control group (P<0.01). T_1/2 of plasma baicalin in the model group was significantly extended to (11.09±1.84) h, with clearance dropping to 61.78% of that of the normal control group (P<0.01). The above results suggested that baicalin had protective effects on the liver of IC rats, accompanied by significantly increased in vivo exposure, delayed in vivo clearance and markedly alterative pharmacokinetic characteristics. This study provides a theoretical basis for further development of baicalin as a feasible drug for treating IC.

Iron depletion induces hepatic secretion of biliary lipids and glutathione in rats

Iron depletion (ID) has been shown to induce the liver expression of Cyp7a1, the rate-limiting enzyme initiating conversion of cholesterol to bile acids (BA), although the effect on bile acids metabolism and bile production is unknown. Therefore, we investigated changes in bile secretion and BA synthesis during diet-induced iron depletion (ID) in rats. ID increased bile flow along with augmented biliary excretion of bile acids, glutathione, cholesterol and phospholipids. Accordingly, we found transcriptional upregulation of the Cyp7a1, Cyp8b1, and Cyp27a1 BA synthetic enzymes, as well as induction of the Abcg5/8 cholesterol transporters in ID rat livers. In contrast, intravenous infusion of ³H-taurocholate failed to elicit any difference in biliary secretion of this compound in the ID rats. This corresponded with unchanged expression of canalicular rate-limiting transporters for BA as well as glutathione. We also observed that ID substantially changed the spectrum of BA in bile and decreased plasma concentrations of BA and cholesterol. Experiments with differentiated human hepatic HepaRG cells confirmed human CYP7A1 orthologue upregulation resulting from reduced iron concentrations. Results employing a luciferase reporter gene assay suggest that the transcriptional activation of the CYP7A1 promoter under ID conditions works independent of farnesoid X (FXR), pregnane X (PXR) and liver X (LXRα) receptors activation. It can be concluded that this study characterizes the molecular mechanisms of modified bile production as well as cholesterol as along with BA homeostasis during ID. We propose complex upregulation of BA synthesis, and biliary cholesterol secretion as the key factors affected by ID.

Upregulation of PDZK1 by Calculus Bovis Sativus May Play an Important Role in Restoring Biliary Transport Function in Intrahepatic Cholestasis

Intrahepatic cholestasis is a main cause of hepatic accumulation of bile acids leading to liver injury, fibrosis, and liver failure. Our previous studies proved that Calculus Bovis Sativus (CBS) can restore biliary transport function through upregulating the multidrug resistance-associated protein 2 (MRP2) and breast cancer resistance protein (BCRP) in 17α-ethynylestradiol- (EE-) induced intrahepatic cholestasis rats. The regulation mechanism of CBS on these transporters, however, remains unclear. This study was designed to evaluate the possible relationship between the effect of CBS on transport activities and the regulation of CBS on the expression of PDZK1, a mainly scaffold protein which can regulate MRP2 and BCRP. Intrahepatic cholestasis model was induced in rats with injection of EE for five consecutive days and then the biliary excretion rates and cumulative biliary excretions were measured. The mRNA and protein expression levels of PDZK1 were detected by reverse transcription-quantitative real-time polymerase chain reaction, western blot, and immunohistochemical analysis. When treated with CBS, cumulative biliary excretions and mRNA and protein expressions of PDZK1 were significantly increased in intrahepatic cholestasis rats. This study demonstrated that CBS exerted a beneficial effect on EE-induced intrahepatic cholestasis rats by restoring biliary transport function, which may result from the upregulation of PDZK1 expression.

Up-regulation of BSEP and MRP2 by Calculus Bovis administration in 17α-ethynylestradiol-induced cholestasis: Involvement of PI3K/Akt signaling pathway

ETHNOPHARMACOLOGY RELEVANCE

Calculus Bovis, also known as Niuhuang, is a rare traditional Chinese medicine that has been widely used in China for 2000 years in pharmacology for sedation, anti-spasm, relieving fever, diminishing inflammation and recovering gallbladder functions.

AIM OF THE STUDY

This study aimed to investigate the choleretic potential and molecular responses in rats to Calculus Bovis (CB) administration after 17α-ethynylestradiol (EE)-induced cholestasis.

MATERIAL AND METHODS

CB (50 and 100mg/kg per day) was intragastrically (i. g.) given to experimental rats for five consecutive days in coadministration with EE (5mg/kg daily for five days, s.c.). The levels of serum biomarkers were determined biochemically. The histopathology of the liver tissue was evaluated. Expression of bile salt export pump (BSEP) and multidrug resistance-associated protein 2 (MRP2) were studied by western blot and immunohistochemical assay. The expression of Akt and phospho-Akt (pAkt) were also measured by western blot.

RESULTS

In response to EE, CB treatment significantly prevented an increase in serum levels of alanine aminotransferase (ALT), alkaline phosphatase (ALP), gamma glutamyltransferase (GGT) and total bilirubin (TBIL). CB treatment also repaired tissue lesions caused by EE. Western blots showed that EE significantly decreased the protein expression of BSEP and MRP2. EE also dramatically increased levels of pAkt and decreased levels of Akt. Compared to the EE group, CB treatment increased levels of hepatic BSEP and MRP2 while pAkt levels decreased and Akt levels increased. Immunohistochemistry also indicated that EE decreased the expression of BSEP and MRP2. LY294002 is a selective PI3K inhibitor and showed similar beneficial effects as CB. Decreased expression of BSEP and MRP2 caused by EE were also prevented by LY294002 treatment.

CONCLUSION

Calculus Bovis administration can alleviate liver injury and up-regulate the expression of BSEP and MRP2 in 17α-ethynylestradiol-induced cholestasis by a mechanism that may involve inhibiting the activated PI3K/Akt signaling pathway.

Boldine enhances bile production in rats via osmotic and farnesoid X receptor dependent mechanisms

Boldine, the major alkaloid from the Chilean Boldo tree, is used in traditional medicine to support bile production, but evidence to support this function is controversial. We analyzed the choleretic potential of boldine, including its molecular background. The acute- and long-term effects of boldine were evaluated in rats either during intravenous infusion or after 28-day oral treatment. Infusion of boldine instantly increased the bile flow 1.4-fold in healthy rats as well as in animals with Mrp2 deficiency or ethinylestradiol induced cholestasis. This effect was not associated with a corresponding increase in bile acid or glutathione biliary excretion, indicating that the effect is not related to stimulation of either bile acid dependent or independent mechanisms of bile formation and points to the osmotic activity of boldine itself. We subsequently analyzed bile production under conditions of changing biliary excretion of boldine after bolus intravenous administration and found strong correlations between both parameters. HPLC analysis showed that bile concentrations of boldine above 10 μM were required for induction of choleresis. Importantly, long-term pretreatment, when the bile collection study was performed 24-h after the last administration of boldine, also accelerated bile formation despite undetectable levels of the compound in bile. The effect paralleled upregulation of the Bsep transporter and increased biliary clearance of its substrates, bile acids. We consequently confirmed the ability of boldine to stimulate the Bsep transcriptional regulator, FXR receptor. In conclusion, our study clarified the mechanisms and circumstances surrounding the choleretic activity of boldine.

New insights in the biology of ABC transporters ABCC2 and ABCC3: impact on drug disposition

INTRODUCTION

For the elimination of environmental chemicals and metabolic waste products, the body is equipped with a range of broad specificity transporters that are present in excretory organs as well as in several epithelial blood-tissue barriers.

AREAS COVERED

ABCC2 and ABCC3 (also known as MRP2 and MRP3) mediate the transport of various conjugated organic anions, including many drugs, toxicants and endogenous compounds. This review focuses on the physiology of these transporters, their roles in drug disposition and how they affect drug sensitivity and toxicity. It also examines how ABCC2 and ABCC3 are coordinately regulated at the transcriptional level by members of the nuclear receptor (NR) family of ligand-modulated transcription factors and how this can be therapeutically exploited.

EXPERT OPINION

Mutations in both ABCC2 and ABCC3 have been associated with changes in drug disposition, sensitivity and toxicity. A defect in ABCC2 is associated with Dubin-Johnson syndrome, a recessively inherited disorder characterized by conjugated hyperbilirubinemia. Pharmacological manipulation of the activity of these transporters can potentially improve the pharmacokinetics and thus therapeutic activity of substrate drugs but also affect the physiological function of these transporters and consequently ameliorate associated disease states.

Changes in pharmacokinetic profiles of acetaminophen and its glucuronide after pretreatment with combinations of N-acetylcysteine and either glycyrrhizin, silibinin or spironolactone in rat

1. The present study was to investigate the effects of giving N-acetylcysteine (NAC) alone and in combination with either glycyrrhizin (GL), silibinin (SIB) or spironolactone (SL) on the plasma pharmacokinetic (PK) profiles, hepatic exposure, biliary excretion and urinary excretion of acetaminophen (APAP) and its major metabolite, acetaminophen glucuronide (AG). 2. Groups of rats (n = 5) were pretreated with oral doses of either NAC, NAC + GL, NAC + SIB or NAC + SL on five occasions every 12 h. At 1 h, after the last dose, they received APAP (200 mg/kg) by intraperitoneal injection. Blood, bile, liver and urine samples were collected at various times after APAP injection and analyzed for APAP and AG by HPLC. NAC alone and NAC + SIB did not significantly change the PK profiles of APAP and AG. In contrast, NAC + GL decreased the biliary excretion of APAP and AG leading to accumulation of APAP in the liver and systemic circulation whereas NAC + SL [multidrug resistance associated 2 (Mrp2) inducer] increased the biliary excretion of AG and decreased the hepatic exposure to APAP and AG. 3. Our results suggest that Mrp2 inhibitor GL should be discouraged with NAC to treat APAP hepatotoxicity. Such PK drug-drug interactions should be considered in the treatment of APAP-induced liver injury.

Endothelin-1 and -3 induce choleresis in the rat through ETB receptors coupled to nitric oxide and vagovagal reflexes

We have reported previously that centrally applied ET (endothelin)-1 and ET-3 induce either choleresis or cholestasis depending on the dose. In the present study, we sought to establish the role of these endothelins in the short-term peripheral regulation of bile secretion in the rat. Intravenously infused endothelins induced significant choleresis in a dose-dependent fashion, ET-1 being more potent than ET-3. Endothelins (with the exception of a higher dose of ET-1) did not affect BP (blood pressure), portal venous pressure or portal blood flow. ET-1 and ET-3 augmented the biliary excretion of bile salts, glutathione and electrolytes, suggesting enhanced bile acid-dependent and -independent bile flows. ET-induced choleresis was mediated by ET(B) receptors coupled to NO and inhibited by truncal vagotomy, atropine administration and capsaicin perivagal application, supporting the participation of vagovagal reflexes. RT (reverse transcription)-PCR and Western blot analysis revealed ETA and ET(B) receptor expression in the vagus nerve. Endothelins, through ET(B) receptors, augmented the hepatocyte plasma membrane expression of Ntcp (Na⁺/taurocholate co-transporting polypeptide; Slc10a1), Bsep (bile-salt export pump; Abcb11), Mrp2 (multidrug resistance protein-2; Abcc2) and Aqp8 (aquaporin 8). Endothelins also increased the mRNAs of these transporters. ET-1 and ET-3 induced choleresis mediated by ET(B) receptors coupled to NO release and vagovagal reflexes without involving haemodynamic changes. Endothelin-induced choleresis seems to be caused by increased plasma membrane translocation and transcriptional expression of key bile transporters. These findings indicate that endothelins are able to elicit haemodynamic-independent biological effects in the liver and suggest that these peptides may play a beneficial role in pathophysiological situations where bile secretion is impaired.

Effects of 17α-ethynylestradiol-induced cholestasis on the pharmacokinetics of doxorubicin in rats: reduced biliary excretion and hepatic metabolism of doxorubicin

1. Since the prevalent hormonal combination therapy with estrogen analogues in cancer patients has frequency and possibility to induce the cholestasis, the frequent combination therapy with 17α-ethynylestradiol (EE, an oral contraceptive) and doxorubicin (an anticancer drug) might be monitored in aspect of efficacy and safety. Doxorubicin is mainly excreted into the bile via P-glycoprotein (P-gp) and multidrug resistance-associated protein 2 (Mrp2) in hepatobiliary route and metabolized via cytochrome P450 (CYP) 3A subfamily. Also the hepatic Mrp2 (not P-gp) and CYP3A subfamily levels were reduced in EE-induced cholestatic (EEC) rats. Thus, we herein report the pharmacokinetic changes of doxorubicin with respect to the changes in its biliary excretion and hepatic metabolism in EEC rats. 2. The pharmacokinetic study of doxorubicin after intravenous administration of its hydrochloride was conducted along with the investigation of bile flow rate and hepatobiliary excretion of doxorubicin in control and EEC rats. 3. The significantly greater AUC (58.7% increase) of doxorubicin in EEC rats was due to the slower CL (32.9% decrease). The slower CL was due to the reduction of hepatic biliary excretion (67.0% decrease) and hepatic CYP3A subfamily-mediated metabolism (21.9% decrease) of doxorubicin. These results might have broader implications to understand the altered pharmacokinetics and/or pharmacologic effects of doxorubicin via biliary excretion and hepatic metabolism in experimental and clinical estrogen-induced cholestasis.

Cholestatic effect of epigallocatechin gallate in rats is mediated via decreased expression of Mrp2

Epigallocatechin gallate (EGCG) has been shown to be protective in various experimental models of liver injury, although opposite effects have also been reported. Since its effect on biliary physiology has not been thoroughly investigated, the present study evaluated effect of EGCG on bile flow and bile acid homeostasis in rats. Compared to controls, EGCG treatment decreased bile flow by 23%. Hepatic paracellular permeability and biliary bile acid excretion were not altered by EGCG administration, but biliary glutathione excretion was reduced by 70%. Accordingly, the main glutathione transporter on the hepatocyte canalicular membrane, multidrug resistance-associated protein 2 (Mrp2), was significantly decreased at the protein level. EGCG administration also doubled plasma bile acid levels compared to controls. While protein levels of the main hepatic bile acid transporters were unchanged, the rate-limiting enzyme in the bile acid synthesis, Cyp7a1, was significantly increased by EGCG. Enhanced bile acid synthesis in these animals was also confirmed by a 2-fold increase in plasma marker 7α-hydroxy-4-cholesten-3-one. In contrast, EGCG markedly downregulated major bile acid transporters (Asbt and Ostα) and regulatory molecules (Shp and Fgf15) in the ileum. When EGCG was coadministered with ethinylestradiol, a potent cholestatic agent, it did not show any additional effect on the induced cholestasis. This study shows ability of EGCG to raise plasma bile acid concentrations, mainly through Cyp7a1 upregulation, and to decrease bile production through reduction in Mrp2-mediated bile acid-independent bile flow. In conclusion, our data demonstrate that under certain conditions EGCG may induce cholestasis.

Induction of hepatic multidrug resistance-associated protein 3 by ethynylestradiol is independent of cholestasis and mediated by estrogen receptor

Multidrug resistance-associated protein 3 (Mrp3; Abcc3) expression and activity are up-regulated in rat liver after in vivo repeated administration of ethynylestradiol (EE), a cholestatic synthetic estrogen, whereas multidrug resistance-associated protein 2 (Mrp2) is down-regulated. This study was undertaken to determine whether Mrp3 induction results from a direct effect of EE, independent of accumulation of any endogenous common Mrp2/Mrp3 substrates resulting from cholestasis and the potential mediation of estrogen receptor (ER). In in vivo studies, male rats were given a single, noncholestatic dose of EE (5 mg/kg s.c.), and basal bile flow and the biliary excretion rate of bile salts and glutathione were measured 5 hours later. This treatment increased Mrp3 mRNA by 4-fold, detected by real-time polymerase chain reaction, despite the absence of cholestasis. Primary culture of rat hepatocytes incubated with EE (1-10 µM) for 5 hours exhibited a 3-fold increase in Mrp3 mRNA (10 µM), consistent with in vivo findings. The increase in Mrp3 mRNA by EE was prevented by actinomycin D, indicating transcriptional regulation. When hepatocytes were incubated with an ER antagonist [7α,17β-[9-[(4,4,5,5,5-Pentafluoropentyl)sulfinyl]nonyl]estra-1,3,5(10)-triene-3,17-diol (ICI182/780), 1 µM], in addition to EE, induction of Mrp3 mRNA was abolished, implicating ER as a key mediator. EE induced an increase in ER-α phosphorylation at 30 minutes and expression of c-Jun, a well-known ER target gene, at 60 minutes, as detected by Western blotting of nuclear extracts. These increases were prevented by ICI182/780. In summary, EE increased the expression of hepatic Mrp3 transcriptionally and independently of any cholestatic manifestation and required participation of an ER, most likely ER-α, through its phosphorylation.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

The subfamily of ABCC transporters consists of 13 members in mammals, including the multidrug resistance-associated proteins (MRPs), sulfonylurea receptors (SURs), and the cystic fibrosis transmembrane conductance regulator (CFTR). These proteins play roles in chemical detoxification, disposition, and normal cell physiology. ABCC transporters are expressed differentially in the liver and are regulated at the transcription and translation level. Their expression and function are also controlled by post-translational modification and membrane-trafficking events. These processes are tightly regulated. Information about alterations in the expression of hepatobiliary ABCC transporters could provide important insights into the pathogenesis of diseases and disposition of xenobiotics. In this review, we describe the regulation of hepatic ABCC transporters in humans and rodents by a variety of xenobiotics, under disease states and in genetically modified animal models deficient in transcription factors, transporters, and cell-signaling molecules.

Altered integrity and decreased expression of hepatocyte tight junctions in rifampicin-induced cholestasis in mice

Rifampicin is a well-known hepatotoxicant, but little is known about the mechanism of rifampicin-induced hepatotoxicity. The aim of this study was to characterize the expression and localization of hepatocyte tight junctions in rifampicin-induced cholestasis in mice. Cholestasis was induced by administration of rifampicin (200 mg/kg) for 7 consecutive days or treatment with a single dose of rifampicin (200 mg/kg) by gastric intubation. The expression of mRNA for hepatic zonula occludens (ZO)-1, ZO-2, ZO-3, occludin and claudin-1 was determined using RT-PCR. Localization of ZO-1 and occludin was detected using immunofluorescence. Results showed that there was an 82-fold increase in the conjugated bilirubin in serum in rifampicin-treated mice. In addition, an 8-fold increase in total bile acid in serum was observed after a seven-day administration of rifampicin. The integrity of hepatocyte ZO-1 and occludin was altered by a seven-day administration of rifampicin. Importantly, the integrity and intensity of hepatocyte tight junctions were altered as early as 30 min after a single dose of rifampicin. The expression of hepatic ZO-1 and ZO-2 mRNA was significantly decreased, beginning as early as 30 min and remaining a lower level 12 h after a single dose of rifampicin. Taken together, these results suggest that the altered integrity and internalization of hepatocyte tight junctions are associated with rifampicin-induced cholestasis.

Hepatic drug transporters and nuclear receptors: Regulation by therapeutic agents

The canalicular membrane represents the excretory pole of hepatocytes. Bile is an important route of elimination of potentially toxic endo- and xenobiotics (including drugs and toxins), mediated by the major canalicular transporters: multidrug resistance protein 1 (MDR1, ABCB1), also known as P-glycoprotein, multidrug resistance-associated protein 2 (MRP2, ABCC2), and the breast cancer resistance protein (BCRP, ABCG2). Their activities depend on regulation of expression and proper localization at the canalicular membrane, as regulated by transcriptional and post-transcriptional events, respectively. At transcriptional level, specific nuclear receptors (NR)s modulated by ligands, co-activators and co-repressors, mediate the physiological requirements of these transporters. This complex system is also responsible for alterations occurring in specific liver pathologies. We briefly describe the major Class II NRs, pregnane X receptor (PXR) and constitutive androstane receptor (CAR), and their role in regulating expression of multidrug resistance proteins. Several therapeutic agents regulate the expression of relevant drug transporters through activation/inactivation of these NRs. We provide some representative examples of the action of therapeutic agents modulating liver drug transporters, which in addition, involve CAR or PXR as mediators.

Hepatocyte-specific ablation of Foxa2 alters bile acid homeostasis and results in endoplasmic reticulum stress

Production of bile by the liver is crucial for the absorption of lipophilic nutrients. Dysregulation of bile acid homeostasis can lead to cholestatic liver disease and endoplasmic reticulum (ER) stress. We show by global location analysis ('ChIP-on-chip') and cell type-specific gene ablation that the winged helix transcription factor Foxa2 is required for normal bile acid homeostasis. As suggested by the location analysis, deletion of Foxa2 in hepatocytes in mice using the Cre-lox system leads to decreased transcription of genes encoding bile acid transporters on both the basolateral and canalicular membranes, resulting in intrahepatic cholestasis. Foxa2-deficient mice are strikingly sensitive to a diet containing cholic acid, which results in toxic accumulation of hepatic bile salts, ER stress and liver injury. In addition, we show that expression of FOXA2 is markedly decreased in liver samples from individuals with different cholestatic syndromes, suggesting that reduced FOXA2 abundance could exacerbate the injury.